Pens that have decorative designs to perform vision effects are known in the arts. One prior art is found in Taiwan Patent Application No. 093217635, which discloses a pen barrel capable of demonstrating a variety of patterns. The pen barrel mainly comprises a transparent outer tube 10 and an inner tube 20 received within the outer tube 10. Referring to the FIG. 6, the outer tube 10 has an identification area 11 defined by a plurality of parallel protrusions 12 with arc-shaped cross-section. And the inner tube 20 has pattern area 21 with a plurality of patterns 211, 212, 213 defined by different sets of stripes. Each set of stripes is paralleled and staggered with others, but the patterns defined are overlapped. Thus, a user can see specific one of the patterns 211, 212, 213 in a time via a specific visual angle through the protrusions 12.
It would be desirable to provide a pen with a barrel performing other vision effect.